Originally developed for early TV cameras, electron multipliers are of considerable importance in low-signal light detection, night vision and mass spectrometry, and in instrumentation for high-energy physics. They provide an analogous function to the erbium doped fibre amplifier in optical communications, namely low-noise, high gain pre-detector amplification. There are a wide variety of different configurations, including discrete dynode devices and mesh multipliers, and continuous dynode devices such as microchannel plates and channeltron multipliers. Variants such as the Gas Electron Multiplier (GEM) are used for particle detection. Some types (e.g. microchannel plates) amplify photoelectrons from an image, while others (e.g. channeltrons) are single-channel devices for instrumentation.
Many materials have been investigated as secondary emissive layers in electron multipliers. Discrete dynode devices typically use metal or semiconductor surfaces, while channeltron devices use reduced lead silicate glasses and microchannel plates use fused arrays of glass tubes or anodically etched alumina. Many other materials have been investigated, including Si, SiO2, Si3N4 (Fijol 1991) and thin film diamond (Beetz 1991).
The work function of the material determines the secondary electron yield (which is typically in the range 1.5-8) and the energy required for peak electron emission (which is typically of the order of 100 V-200 V). A cascade of emission events involving total voltages of more than 1 kV is normally required to obtain a high overall gain. In a discrete dynode device, the necessary voltages may be provided from a single voltage source by a chain of biasing resistors, which are normally external to the device.
If the material system can withstand such a large voltage, gains of up to 106 are possible. In a discrete dynode device, the emitting surfaces are spaced by insulator material. In a continuous dynode device, the voltage is dropped along the emitting material itself, which must be partially conducting to be able to re-supply the secondary electrons from the external circuit. This dual function is achieved by using an activated surface backed by a partially conducting layer.
Microelectromechanical systems (MEMS) technologies potentially offer considerable advantages for electron multipliers, since they allow the fabrication of very high aspect ratio structures such as channels. For example, the German LIGA (standing for Lithographie, Galvanoformung, Abformung) process is a method of fabricating near-vertical deep features by electroplating in a mould formed by exposure of resist to synchrotron radiation (Ehrfeld et al. 1991; Guckel et al. 1998). Features with arbitrary shapes may be formed, in an orientation normal to the substrate plane; however, the process is expensive, and the features are formed in non-silicon materials such as electroplated metal. Although these materials can be modified with additional surface coatings, the choice of coatings that may be used for secondary electron emission is restricted.
Anisotropic etching such as that described in Bean (Bean 1978) is a method of fabricating high aspect ratio structures in silicon, which potentially allows the fabrication of silicon-based surfaces for secondary emission. However, the range of possible features is limited, because the process forms structures that are bounded by specific crystal planes (typically, the <111> plane). As a result, it is not possible to form arbitrary features orientated normal to the substrate plane.
Deep reactive ion etching (DRIE) is an alternative and more flexible method of forming near vertical features in silicon. The process involves the cyclic use of alternate etching and passivation steps (U.S. Pat. No. 5,501,893; Hynes et al. 1999). In the etch step, a reactive plasma derived from SF6 is used to remove silicon. Although the etching is actually isotropic, lateral erosion is prevented by deposition of a polymer (derived from CxFy) in the passivation step. To re-initiate etching, fluorine radicals first etch the base of the passivation, and then the silicon itself. High etch rates (of the order of 4 μm/min) and depths of >500 μm may be achieved, with wall angles of >89°. The selectivity to oxide is extremely high, so that a glass or oxide layer can act as an etch stop.
There has been some effort to apply the methods above to the fabrication of electron multipliers. For example, U.S. Pat. No. 4,990,827 describes a discrete dynode multiplier, which is to be formed by the LUGA process. However, it is unclear whether the device was actually fabricated, and issues relating to suitable materials do not seem to have been addressed. LIGA has been used to fabricate slant-hole detectors for high-energy physics (Fukuda et al. 1999; Inoue et al. 2000). In both these cases, the detectors were through-wafer devices.
Microchannel plates have also been formed, by etching silicon, in an attempt to develop low-cost night-vision equipment. For example, U.S. Pat. No. 4,482,836, U.S. Pat. No. 5,618,217, U.S. Pat. No. 6,384,519 and U.S. Pat. No. 5,997,713 all describe microchannel plates constructed from stacked Si wafers which have been etched by a variety of methods including anisotropic etching, and electrochemical etching. U.S. Pat. No. 5,544,772 describes a simpler through-wafer multiplier, in which a single Venetian blind electrode is fabricated by etching of silicon. U.S. Pat. No. 5,568,013 describes an alternative configuration, in which amplification takes place within etched channels, which lie in the plane of the silicon wafer. Finally, the use of DRIE for forming high-aspect ratio channels in a through-wafer multiplier has been extensively explored (Shank 1995; Beetz 2000). Arrays of Faraday cup detectors have also been formed by DRIE (Darling 2002).
There is considerable demand for small single-channel multipliers to act as detectors in vacuum gauges and miniature vacuum instruments such as mass spectrometers. The latter are rapidly assuming importance as portable drugs and explosives detectors in the global effort against crime and terrorism. Microengineered mass filters have now been demonstrated as crossed-field, ion trap, quadrupole and time-of-flight devices (Badman et al. 2000); however, very little attention has been paid to the equally important problem of detector miniaturisation and integration.
In fact, detectors for miniature analytic instruments differ from microchannel plates in an important respect. To obtain sufficient selectivity from a microengineered mass filter, the ions normally travel parallel to the wafer plane, so that a sufficiently long ion path is achieved. Any compatible detector should therefore have a similar format, i.e. in-plane ion and electron paths. In such an arrangement, it is then possible to develop complex electrode structures by surface patterning and etching, and to modify or coat the exposed surfaces to enhance material properties such as secondary electron emissivity or resistivity. However, to date no such structures are available.
Increasingly, miniature analytic instruments are being constructed from silicon compatible materials using planar processing. However, the electron multiplier devices described above are generally not compatible with such a scheme. Connection to the individual electrodes is also complex, requiring many separate electrical wires. There is therefore a need for a simple method of forming secondary electron multipliers in a compatible format.